Abstract: We have refined vibrational force fields for polypeptides that permit excellent reproduction of the normal mode frequencies of such molecules. This is demonstrated in the present study, in which 80 IR and Raman bands of crystalline triglycine between 1800 and 200 cm-1 are reproduced with an average error of 6 cm-1. A deuterated sample is shown by normal mode analysis to have remained protonated at the C-terminal peptide group. Such results show that normal mode analysis can now provide a rigorous base for spectral studies of conformation in peptides and proteins.